The present invention relates to a hollow golf ball. More particularly, it relates to a hollow golf ball having good shot feel at the time of hitting by making the impact force small, while maintaining excellent flight performance.
Hitherto, there have been mainly produced two types of golf balls. The one is a solid golf ball, such as a two-piece golf ball or three-piece golf ball, and the other is a thread wound golf ball. The solid golf ball, when compared with the thread wound golf ball, has better durability and better flight performance because of a larger initial velocity at the time of hitting and longer flight distance. Therefore, the solid golf ball is generally approved or employed by many golfers, mainly amateur golfers. With regard to enhancement of the flight distance, the development of the golf ball is mainly directed toward solid golf balls rather than thread wound golf balls.
On the other hand, the solid golf ball exhibits a hard and poor shot feel at the time of hitting. It has been known that the flight distance is largely affected by rebound characteristics in the solid golf ball. Recently, in order to improve the shot feel of the solid golf ball, it has been attempted to soften the core of the solid golf ball to reduce the hardness of the golf ball. However, there is the drawback that the rebound characteristics of the golf ball are degraded and the flight performance is reduced, because of the softening of the core.
In order to extend the flight distance of golf balls, it is very important that the rebound characteristics of the golf ball are enhanced. The enhanced rebound characteristics may preferably be obtained by reducing the deformation amount of the golf ball. However, the reduced deformation amount of the golf ball adversely hardens the golf ball and deteriorates the shot feel at the time of hitting. On the other hand, when the golf ball is made soft and its deformation amount is large, the shot feel at the time of hitting is improved, but the rebound characteristics are reduced and the flight performance is reduced. Therefore, it is very difficult to improve both flight performance and shot feel of the conventional solid golf ball.
A main object of the present invention is to provide a hollow golf ball having good shot feel at the time of hitting by making the impact force small, while maintaining excellent flight performance.
In order to solve the problem, the present inventors have proposed to provide a hollow golf ball having good shot feel at the time of hitting, while maintaining excellent flight performance, by employing a hollow core composed of a hollow portion and a hollow core outer layer which enhances the moment of inertia of the resulting golf ball.
Retention of the spin amount of the golf ball can vary depending on the moment of inertia of the golf ball as well as the shape of the dimples. In general, when the moment of a inertia of golf ball is charge, it is difficult to apply spin on the golf ball, and the spin amount is easily maintained. That is, it is difficult to apply spin on the golf ball having a large moment of inertia at the time of hitting by a driver, and thus the flight distance is extended because the golf ball does not create a blown-up trajectory. As is further explained in detail, when a golf ball is hit by a golf club, the lifting power acts on the golf ball, and the partial force of the lifting power in the horizontal direction acts positively to the ball flight direction after the golf ball passes the highest point of the flight curve. That is, it is difficult to apply spin on the golf ball having a larger moment of inertia at the time of hitting by a driver. The lower the spin amount, the smaller the lifting power, and the partial force in the direction of backing the golf ball is small to extend flight distance, from immediately after hitting the golf ball by a driver to the arrival at the highest point of the flight curve of the golf ball. The higher the spin amount (the larger the retention of spin amount), the larger the lifting power acts on the golf ball, and the partial force of the lifting power in the flight direction is larger for extending the flight distance, after the golf ball passes the highest point of the flight curve. The larger the moment of inertia, the smaller the descending angle of the golf ball. Thus the a distance from the point when the golf first drops to the ground to the stop point finally reached by the golf ball is extended. Therefore, the larger the moment of inertia of the golf ball, the longer the flight distance of the golf ball, aerodynamically. With approach shots, when the moment of inertia of the golf ball is larger, it is easy to stop the golf ball because of easily retaining the spin amount.
The present inventors have further studied the hollow golf ball for shot feel at the time of hitting and rebound characteristics in detail, and then different results from conventional solid golf balls have been obtained. That is, in the case of the hollow golf ball, golf balls have the same deformation amount but may show different rebound characteristics (flight distance) and impact force (shot feel). The reason for this is believed to be as follows. The hollow golf ball has a hollow portion, thus a hollow core may deform also in a direction of flexure. Therefore, the deformation amount of the hollow golf ball can be determined depending on the compressive modulus and flexural modulus of the core composition, and the diameter of the hollow portion. The hollow golf ball shows different behavior from the solid golf ball of which the deformation amount can be only determined by the compressive modulus of the core composition.
In order to improve both rebound characteristics and shot feel by using the specific characteristics of the hollow golf balls as explained above, the present inventors have taken note of the deformation amount and natural frequency of the golf ball. The deformation amount has been considered to effect both the rebound characteristics and shot feel at time of hitting. The natural frequency is estimated to effect the impact force, that is, the reactive force given to a golf club upon hitting a golf ball. As a result, the inventors have found that a hollow golf ball having superiority in both rebound characteristics and shot feel can be obtained by adjusting the difference between the deformation amount (Y mm) and the secondary natural frequency, (X kHz) within a certain range. The deformation amount is determined from applying from an initial load of 10 kgf to a final load of 130 kgf on the hollow golf ball.